Theoretically, a fuel cell outputs electric power along with water generated by reaction of oxygen and hydrogen only, thus it is a clean energy source without being a burden on the environment. The electrolyte materials used in fuel cell mainly include: polymer solid electrolyte type fuel cell (“PEFC” for short), phosphoric acid fuel cell (“PAFC” for short), molten carbonate fuel cell (“MCFC” for short), solid oxide fuel cell (“SOFC” for short), and the like. Among them, SOFC uses an ion conductive metal oxide as the electrolyte, and uses a mix conductive oxide as (cathode) the air electrode.
Solid electrolyte material is a key material used in the applications such as fuel cell and oxygen sensor and the like in the fields like automobile. At present, mature solid electrolyte materials in the world include oxide materials like yttrium-stabilized zirconia (“YSZ” for short) and the like, which are used for fuel cell, oxygen sensor, and etc. Such materials, with a operation temperature of usually about 1000° C., have excellent performances and a relatively lower price. However, the high operation temperature of 1000° C. causes difficulty in manufacturing and operating devices, whilst the chemical reaction between YSZ and member materials also results in deterioration of materials due to long-term use under high temperature, and makes it difficult to perform processes like material joining, etc. On the other hand, electrolyte materials used for automobile exhaust gas sensors need to overcome problems such as thermal shock failure, long start-up time, etc. In recent years, countries in the world have paid attention to the development of materials which have high ionic conductivity at low temperatures. Particularly, for devices with high power output at a relatively low temperature, solid electrolyte materials are required to have high ionic conductivity and high stability at low temperatures. Besides, electrode materials for the air side of a fuel cell demand for oxide materials with high mix conductivity.
So far, solid electrolyte materials developed and disclosed include lanthanum gallate oxides series (Patent Literature 1), a mixed system of stable bismuth oxide series and stable zirconia (Patent Literature 2) and cerium oxide series composite oxides (Patent Literatures 3-6).
Cerium oxide (CeO2), zirconia, bismuth oxide and the like are all ion conductive materials with fluorite structure. High oxygen-ion conductivity is obtained by doping with low-valence metal elements to form oxygen deficiency (vacancy). For example, Patent Literature 3 teaches to further dope cerium oxide with other 1-valence or 2-valence elements on the basis of doping it with 3-valence rare earth element, such as doping cerium oxide with yttrium oxide. In Patent Literature 4, a high ionic conductivity is obtained by partly replacing cerium atoms in cerium oxide with lanthanum atoms of large ion radius, and replacing cerium atoms with strontium (Sr) or barium (Ba) of 2-valence to increase disorder in the oxygen vacancy. Patent Literature 5 teaches that replacing the position of 4-valence cerium with greater cations of 2-valence and 3-valence results in oxygen deficiency, and in a greater crystallization stress, and a high ionic conductivity is thus obtained. Patent Literature 6 teaches a high oxygen-ion conductivity at a temperature of 800° C. or lower and an oxygen partial pressure of 10-30-10-15 atmospheric pressure (atm) is obtained by doping cerium oxide with elements such as ytterbium (Yb), yttrium (Y), gadolinium (Gd), samarium (Sm), neodymium (Nd), Lanthanum (La) and the like.
However, in case that a metal oxide is used as the cathode and the electrolyte material of fuel cells (SOFC), chemical reactions between three phase materials of gas/electrode/electrolyte often occur, in which gas, ion and electron participate simultaneously. To facilitate the above reactions, solid electrolyte and electrode having fibrous metal oxide have been invented, as shown in Patent Literatures 7 and 8.
When the cerium oxide series composite oxides as described in Patent Literatures 3-5 are doped with alkaline earth metals, carbonates are easily generated under the effect of ambient atmosphere, which results in a decrease in conductivity, and in turn arouses the problem of structural stability of the solid electrolyte materials during usage. In general, addition of 3-valence rare earth elements or 2-valence alkaline earth metal elements into oxides of 4-valence cerium can all increase the concentration of oxygen vacancy, but excessive doping may lead to generation of other compounds, and thus cause a decrease in conductivity. Furthermore, 4-valence cerium ion Ce4+ in cerium oxide will be reduced to 3-valence cerium ion Ce3+ at a high temperature and a reducing atmosphere to give rise to electronic conductivity, and thereby reduce ionic conductivity and the efficiency of fuel cells. Besides, the reduction reaction also leads to crack of the cerium oxide solid electrolyte material, and thus failure.
Hence, although various composite oxide solid electrolyte materials have been developed up to the present, demands for fuel cells (SOFC) with high ionic conductivity and high power output under low operation temperature can still hardly be met.
Patent Literature 1: Japanese Publication JP2004-339035;
Patent Literature 2: Japanese Patent JP59-18271;
Patent Literature 3: Japanese Patent JP09-2873;
Patent Literature 4: Japanese Patent JP2000-109318;
Patent Literature 5: Japanese Patent JP2004-87271;
Patent Literature 6: Japanese Patent JP2004-143023;
Patent Literature 7: Japanese Patent JP2006-244810;
Patent Literature 8: Japanese Patent JP2009-197351.